The use of a jaw clutch for selectively connecting a main or driven shaft with either of two constantly rotating driving gears on countershafts paralleling the driven shaft has been described in German Pat. No. 1,630,452. Clutches of this nature are also available under the designation MACK TRL-107, for example. In order to avoid gear clashing, such systems must be provided with separate synchronizing means.
In constant-mesh gear transmission, especially in those of the synchromesh type, it is known to mount a sliding gear in a non-rotatable manner (through keying or splining) on a driven shaft adjacent a coaxial driving gear of the same pitch and diameter, the external teeth of the sliding gear being in mesh with internal gears of a surrounding sleeve which can be axially shifted to bridge the two gears. The sliding gear and its sleeve are releasably coupled together with the aid of one or more spring-loaded ball checks allowing their joint displacement by a shift fork linked with the sleeve. Upon such displacement from a disengaged position toward an engaged position, a frustoconical inner surface of the sliding gear comes into contact with a correspondingly tapered outer surface of the driving gear whereby the assembly of sliding gear and sleeve is frictionally entrained and brought into synchronism with the driving gear. At this point the sleeve can be axially shifted with reference to the sliding gear, against the yieldable retaining force of the ball checks, into its bridging position to establish a positive driving connection between the two gears.
A synchromesh clutch of this type, which obviously cannot be used with a countershaft, has been described in "Principles of Automotive Vehicles", Manual TM9-8000 and TO36A-1-76, published January 1956 by the Department of the Army and the Air Force, pages 284-288.